Electronic and micromechanical devices are formed by patterning successive layers on a substrate using lithography. The patterns are formed by applying a layer of photoresist to a surface. Light is then passed through a patterned imaging plate, such as a mask or reticle, to expose the photoresist in patterns that correspond to the desired features on the substrate. A developer is applied and the photoresist is etched away leaving only the features in a pattern corresponding to the pattern on the mask. As the size of the features, such as parts of transistors, decreases, there are more features on the same size mask and the mask designs becomes more complex.
The spacer photolithography patterning process uses a combination of photolithography masks in order to achieve a designed layout pattern on a layer of photoresist. Depending on the process choice, two or three masks are synthesized based on the drawn layout data. The three masks are used in sequence to expose a single layer of photoresist and are typically a backbone mask, a trim mask, and an optional add mask. The add mask is the simplest to synthesize since it contains areas that are larger than the lithographically challenging regions. The primary function of the add mask is normally to add any large features that cannot be synthesized using the backbone and the trim masks alone.
The backbone mask is the most difficult to synthesize because of the nature of the spacer process that is used in the spacer patterning process. In the spacer process each edge of the backbone mask leads to a structure on the wafer that will either contribute to the design or will be removed using the trim mask. In addition, the mask is still harder to design because it must also be manufacturable. As a result, it is not always obvious how to synthesize the design of a backbone mask that has any shape other than a repetitive grating pattern. One such example is shown in the example below.
The design space for the backbone masks is often restricted to simple one dimensional grating structures with strict limits on pitch variations. In other words, the distance between the lines of the grating is not allowed to significantly vary. Such restrictive design rules simplify the synthesis requirement at the cost of a loss in design flexibility. Simplifying all the shapes to a grating pattern is an extremely restrictive design rule that limits the possibilities of the spacer process.